Native glucagon is a 29 amino acid peptide that regulates blood glucose levels through enhanced synthesis and mobilization of glucose in the liver. Consequently, the suppression of endogenous glucagon action has been a target for the development of drugs to treat conditions characterized by excessive glucose production, such as diabetes.
Glucagon generally functions as a counter-regulatory hormone, opposing the actions of insulin, to maintain the level of blood glucose, particularly in instances of hypoglycemia. However, in some patients with Type 1 or Type 2 diabetes, absolute or relative elevated glucagon levels have been shown to contribute to the hyperglycemic state. Both in healthy control animals as well as in animal models of Type 1 and Type 2 diabetes, removal of circulating glucagon with selective and specific antibodies has resulted in reduction of the glycemic level (Brand et al., Diabetologia 37, 985 (1994); Diabetes 43, [suppl 1], 172A (1994); Am. J. Physiol. 269, E469-E477 (1995); Diabetes 44 [suppl 1], 134A (1995); Diabetes 45, 1076 (1996)). These studies suggest that glucagon antagonism could be useful in glycemic control of diabetes.
Glucagon exerts its action by binding to and activating its receptor, which is part of the glucagon-secretin branch of the 7-transmembrane G-protein coupled receptor family. The receptor functions by an activation of the adenylyl cyclase resulting in increased cAMP levels. Previous reports have identified peptide-based, (see Unson, C. G. et al. (1989) J. Biol. Chem. 264, 789-94, Ahn, J. et al. (2001) J. Peptide Research 58, 151-8 and Ahn J. et al. (2001) J. Med. Chem. 44, 1372-9) as well as nucleotide-based glucagon antagonists (Sloop K. et al. (2004) J. Clinical Invest. 113, 1571-81). Peptide-based inhibition acts at the level of receptor binding while the latter functions by suppressing intracellular mRNA specific to the glucagon receptor.
Inhibitors of the glucagon receptor have been described, and are generally based on the amino acid sequence of glucagon. Several analogues in which one or more amino acids were either deleted or substituted to produce potent antagonists of glucagon receptor have been described, for example, [des His1] [Glu9]-glucagon amide (Unson et al., (1989) Peptides 10, 1171; Post et al., (1993) Proc. Natl. Acad. Sci. USA 90, 1662), des His1, Phe6 [Glu9]-glucagon amide (Azizh et al., (1995) Bioorg. & Med. Chem. Lett. 16, 1849) and Nle9, Ala11,16-glucagon amide (Unson et al. (1994) J. Biol. Chem. 269(17), 12548). Other analogues include substitutions at positions 4 (Ahn J M et al. (2001) J. Pept. Res. 58(2):151-8), 1 (Dharanipragada, R. et al. (1993) Int. J. Pept. Res. 42(1): 68-77) and 4, 5, 12, 17 and 18 in the glucagon sequence (Gysin B et al. 1986. Biochemistry. 25(25):8278-84).
As described herein, high potency glucagon antagonists are provided that represent modifications of the native glucagon peptide. More particularly, the novel glucagon antagonist represent novel chemical modifications of the N-terminus of the native glucagon sequence, producing a highly specific antagonist that exhibits no apparent agonist activity. These compounds can be used in any setting where the suppression of glucagon agonism is desired. In accordance with one embodiment the compounds can be used in the treatment of diabetes.